This invention relates to protective relay circuits and in particular to a circuit for processing electric power system signals in order to detect a fault condition and then to generate a signal for operating the trip coil of a circuit breaker. It is well known in the relay art that protective relays have a tendency to pick up for faults farther away than expected if the offsets due to transients in the fault signals arriving at the relay terminals are not eliminated. This tendency is known in the relay art as "overreach". For a discussion of overreach see The Art and Science of Protective Relaying, Mason, published by John Wiley & Sons, Inc. (1956), pages 82-83, 308-310, and 350-352. Transients in the power system also occur during the normal operation of the electrical power system, for example, when load is being picked up or dropped. Since the above-discussed transients can result in the erroneous detection of a fault condition, it is common in protective relays to filter the measured electrical power system signals in order to remove the transient components therefrom. The filter provides increased security by preventing false tripping of the circuit breaker during certain transient conditions at the expense of an increased time in detecting a fault condition. Since the filter is designed to provide the desired system security for the lowest level of fault signals, there is excessive delay in detecting certain high level fault signals. One prior art solution for reducing the delay in detecting high level fault conditions involves establishing a first threshold level for the measured fault parameter above which the trip coil of the circuit breaker will be instantaneously activated. In this prior art solution there is still excessive delay in detecting intermediate level fault signals less than the first predetermined threshold level. It would be desirable to establish a second threshold level for the fault signal further dividing the fault signal into a low range of levels and an intermediate range of levels and to speed up the generation of the signal for operating the circuit breaker for intermediate levels of the fault signal.
It is, therefore, one object of my invention to provide a protective relay having an improved circuit for generating a signal for controlling the trip coil of a circuit breaker.
It is another object of my invention to provide a protective relay having an improved circuit, including a filter, for monitoring the condition of an electrical system wherein for intermediate fault signal levels, the extent to which the filter determines the delay time to the generation of a tripping coil signal is a highly inverse function of the fault signal level.
Another object of my invention is to provide an improved overcurrent or overvoltage protective relay.
And yet another object of my invention is to provide an improved directional relay.
Another object of my invention is to provide an improved distance relay.